1. Field of the Invention
The present invention relates to a nonaqueous electrolyte for an electrochemical device, and more particularly to an electrolyte additive which is useful to suppress the occurrence of degradation, and an electrochemical device having the same.
2. Description of the Related Art
In recent years, there is an increase tendency to research the energy saving technology. Rechargeable batteries have been popularly applied in several aspects, such as mobile phones, camcorders, and notebooks. The related field has been extensively researched, wherein secondary batteries are more interested. For secondary batteries, the major research is in enhancing the energy density and the cycle life.
In current secondary batteries, lithium ion batteries are developed in 1990. In comparison with traditional batteries using aqueous electrolytes (such as nickel-hydride batteries, nickel-cadmium batteries, and lead acid batteries), lithium ion batteries have high working voltage and energy density. Therefore, people invest a lot of time in researches of lithium ion batteries. However, one of the drawbacks of lithium ion batteries is the capacity fading during repeated charging-discharging cycles. The more the capacity of a lithium ion battery has, the more serious this problem is. Hence, lifetimes of lithium ion batteries need to be further enhanced. One of approaches to enhance the lifetimes of lithium batteries is to modify electrolyte composition by suitable additives.
In the electrolytes of lithium ion batteries, carbonate-based organic compounds are commonly used as solvents. According to their structures and characteristics, they are briefly classified into two groups: one is cyclic carbonates with high dielectric constant and viscosity, such as ethylene carbonate (EC) and propylene carbonate (PC); the other group is linear carbonates with low dielectric constant and viscosity, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). An ideal electrolyte must have high dielectric constant and low viscosity at the same time. Thus, general electrolytes contain a mixture of cyclic carbonates and linear carbonates to obtain the required properties of both dielectric constant and viscosity.
However, EC and PC have the following characteristics, respectively. During the first charging step, EC can form a stable passivation layer, such as a solid electrolyte interface (SEI), on the surface of anode to protect the anode material from exfoliation, while PC can not do the same. However, EC will loss its fluidity under 37° C. (melting point of EC). It will result in poor charging-discharging performances of battery under low temperature. Conversely, PC still has a good fluidity under low temperature arising from its low melting point (−49° C.), but it is prone to produce co-intercalation with lithium ions into graphite layers during a charge process, resulting in detrimental graphite exfoliation. Thus, if the content of PC is too high, it usually results in decreasing the lifetime of a battery.
In order to solve the above problems, a mixture of EC and PC is usually used in commercially available electrolytes to avoid the above drawbacks and enhance the performance of a battery. Except for adjusting the ratio of solvents, using additives is the most effective way to improve lifetime, capacity, low temperature performance and of a battery. Nevertheless, common additives such as vinylene carbonates, sulfites, sulfates, phosphates or derivatives thereof have not only expensive prices, but also barely satisfactory effects.
In this regard, Japanese publication patent 2002-158034 disclosed an acrylic acid compound used as an additive of electrolyte in a lithium ion secondary battery. The additive (acrylic acid) can suppress gas reduction and the decay of anode in the lithium ion secondary battery. Besides, Japanese publication patent 2003-168479 disclosed an acrylic acid compound with at least three acrylic aldehyde groups used as an additive of electrolyte in a lithium ion secondary battery. The compound can form a solid electrolyte interface (SEI) layer by the reduction reaction on the anode. The SEI layer can suppress the degradation of an electrolyte, and to improve cycle life of the battery. In addition, WO 2008/050971 disclosed an acrylic acid compound with a polymerizable double bond used as an additive of electrolyte in a lithium ion battery. The acrylic acid compound also had the effect of forming a SEI layer.
Based on the existing techniques, we desire to develop a novel additive of lithium ion battery which is useful for forming a steady SEI layer on the surface of the carbonaceous material to suppress its exfoliation, thereby further enhancing the lifetime of a lithium ion secondary battery.